1. Field of the Invention
The present invention relates to a circuit for controlling drive current that flows in a stator winding in accordance with a signal from a position detecting element that detects a rotational position of a rotor in a DC brushless motor such as a fan motor.
2. Description of the Prior Art
In a conventional technique concerning a two-phase DC brushless motor such as a fan motor, as disclosed in Japanese unexamined patent publication No. 9-047073 for example, a position detecting element such as a Hall element is provided for detecting a rotational position of a rotor, so that current flowing in a stator winding is controlled in accordance with the output signal of the element.
FIG. 5 shows a conventional two-phase half-wave (unipolar) brushless DC motor drive circuit that is applied to such a DC fan motor. This circuit includes a drive IC 12 (for example, a motor drive IC, BA6811F made by ROHM CO., LTD.) for driving two-phase stator windings 14 and 15, a Hall element 11 as a position detecting element, a diode 13 and a capacitor 16. The drive IC 12 includes an operational amplifier 17, a control circuit 18 and transistors 19 and 20 for phases. The control circuit 18 works so as to turn on the transistor 19 or 20 at an output side when an input voltage of the operational amplifier 17 becomes zero level.
This conventional DC motor drive circuit shows characteristics in which the current flowing in the two-phase stator winding does not increase rapidly just after one of the two transistors is turned on, but the current increases gradually because of a resistance and an inductance of the winding. The time constant due to the resistance R and the inductance L of the winding is represented by L/R. If this time constant is larger than the switching period T of the Hall element, the current flowing in the stator winding does not increase to a level that is sufficient for generating a drive force in the period T. Even if the current increases, it occurs in the latter half of the period T. In the case of a high speed fan motor, the period T becomes short, so the tendency that the current value increases only in the latter half of the period T may become conspicuous.
In order to explain a principle that is a precondition for understanding the present invention, it will be explained first how rotation of a typical brushless motor and switching of the stator winding current contribute a rotational drive force with reference to FIGS. 8A and 8B. The positions indicated by arrows (a)–(f) in FIG. 8A respectively denote times corresponding to positions (a)–(f) of the rotor in FIG. 7. It shows that the current that was flowing in one winding during the period from (c) to (d) is switched to flow in the other winding. Furthermore, FIG. 8A shows a waveform of current that flows in a winding of a fan that has a high rotation speed. In a fan that has a high rotation speed, a thick wire is used for making the winding so as to reduce a resistance R of the winding for increasing the current I that flows in the winding and for increasing a rotation torque. In this fan, since the resistance R of the winding is small, the time constant (L/R) of the winding becomes large, and the current waveform has a shape as shown by a continuous line in FIG. 8A in which the current increases rapidly at the end of the period T. Furthermore, the waveform shown by a dotted line in FIG. 8A is a waveform in the case where there is no delay of the current waveform due to the inductance of the winding. Therefore, when the power supply voltage is E, the peak value Ip2 of the current substantially equal to E/R. Since the time constant of the winding is L/R, the time constant becomes a large value in a fan motor using a winding with a small resistance. As a result, the delay of the current waveform becomes large like the current waveform as shown by the continuous line in FIG. 8A. Although the peak value Ip3 of the current is smaller than Ip2, it becomes a very large value compared with current value at the time (b) that is positioned at the middle of the period. Here, the period T corresponds to the time in which the motor rotates a ¼ turn. The rotor position that corresponds to the time (b) in FIG. 8A is the position in which the stator current is converted into the rotor rotation torque most efficiently as being explained later. In contrast, even if the stator current is increased in the position corresponding to the time (c), it is not converted into the rotation torque efficiently. Therefore, in the fan having the current waveform as shown in FIG. 8A, the current that is supplied to the stator can not be converted into the rotation torque efficiently.
Such stator current that is not converted into the rotation torque is consumed or wasted as heat by portions that are snubber circuits 50 and 51 as shown in FIG. 5. Accordingly, it is required to control the current supply to the stator winding so that the current becomes a peak at the time (b) when it is converted into the rotor rotation torque most efficiently. However, in the conventional control circuit, the stator current value increases in the latter half of the period T like the stator current as shown by the continuous line in FIG. 8A, and a large portion of the current is consumed as heat in the snubber circuit. This means that efficiency of converting an electric energy supplied to the motor into a rotation torque is small.